Fig. 456.—The Swiss lakes Thun and Brienz, formed by deltas at the junction of streams tributary to a steep-walled valley.
Side-delta lakes.—It is characteristic of river drainage that the tributary streams enter the main valley on steeper gradients than the trunk stream at the point of junction. Wherever the difference in velocity of the two streams at the junction is large, and the side stream is charged with sediment, a delta will be formed at the mouth of the tributary stream. Such deltas push out from the shore and may eventually block the main channel so as to form a more or less sausage-shaped expansion of the river—a side-delta lake. Traverse and Big Stone Lakes in the valley of the Warren River in Minnesota have been formed in this way ([Fig. 354], [p. 326]). Lakes Thun and Brienz in the Swiss Alps are of similar origin, the beautiful city of Interlaken being built upon the delta plain over the valley of the earlier river ([Fig. 456]). The Mississippi has similarly been expanded to form Lake Pepin above the delta at the mouth of the Chippewa River.
Fig. 457.—Delta lakes formed at the mouth of the Mississippi through the junction of the levees of radiating distributaries with the shore of the estuary (after Berghaus).
Delta lakes.—A somewhat different type of delta lake has been formed in Louisiana, where the “father of waters” discharges into the gulf. Here the various distributaries radiate from the main channel to produce the “bird-foot” delta type and the toes in this foot by their junction with the banks which outline the ancient estuary, have separated in succession a series of basins that before were in direct connection with the sea ([Fig. 457]). Lake Pontchartrain is the largest of this series, while the so-called Lake Borgne is in process of separation.
Where large deltas push out from the shore into the open sea, the levees which border the individual distributaries are attacked by the waves and their materials are transported by the shore currents and built into barriers. These barriers cut off the re-entrants between neighboring distributaries so as to produce lagoons or lakes ([Fig. 458]).
Fig. 458.—A type of delta lakes formed by levees in part destroyed and built into barriers on the margin of the delta of the Nile (after Supan).
A type of delta lake, which more resembles the side-delta lake above described, has formed at the mouth of the Colorado River, where it enters the Gulf of Lower California. The Imperial Valley lying to the north of this delta is the desiccated floor of the earlier Gulf of Lower California which has been captured from the sea by the delta of the Colorado. The rampart of mountains, by which this valley is surrounded, has cut it off from any water supply derived from clouds, and its waters being no longer renewed from the sea, the region has passed through a period of desiccation which has left the Salton Sink as the only existing remnant of the earlier lagoon. It will be remembered that careless operations in diverting distributaries of the Colorado recently reversed this process so that the waters rose in the valley, and expensive emergency operations were necessary in order to again turn the waters of the Colorado into their accustomed channels.
